This invention relates to a method of producing a dehydrated food product utilizing microwaves. More particularly, this invention relates to a dehydrated food product which is useful as an ingredient for use in instant food and as snack food and a method of producing the same.
Recently, many dehydrated and processed food products, for example, dehydrated and processed vegetables, meat, "tamagoyaki" (fried beaten egg) and "aburage" (fried "tofu"), have been studied and developed as ingredients for use in instant (or convenient) food and the conventional methods have used a drying means such as hot-air drying, vacuum freeze-drying, drying due to high-frequency heating and microwave drying.
However, the dehydrated food product obtained by hot-air drying generally has defects that the food loses a very large part of its original volume after drying and because of this the tissue shrinks and becomes hard and thus rehydration of the dehydrated food product is bad. On the other hand, the dehydrated food product obtained by vacuum freeze-drying is satisfactory to some degree as regards rehydration but there are problems that the drying time is long and the drying process is very expensive to carry out.
Microwave drying does not cause the problems encountered in the above drying methods but there is a defect that it sometimes induces uneven heating in the food to be treated. Thus, in some case, it is difficult to uniformly dehydrate the food to be treated and the obtained dehydrated food product cannot uniformly be rehydrated.
In order to obviate the above defects, for example, regarding the rehydration of dehydrated food product, (e.g., dehydrated "aburage"), the following drying methods are known: "aburage" after being immersed in a seasoning liquor or without immersing therein is pressed with a roller and thereafter is dehydrated by the freeze-drying, hot-air drying or drying due to high-frequency heating. However, these known methods can not completely obviate the above defects. The obtained dehydrated "aburage" product has an uneven shape and a bad appearance, and thus is not suitable for continuous and automatic packing and packaging.
U.S. Pat. No. 3,973,047 discloses a process of dehydrating vegetables by a hot-air drying technique. In this patent, vegetable or vegetable pieces are dehydrated with hot air to a moisture content less than about 30% and are thereafter contacted with air having a dry bulb temperature from about 140.degree. F. to about 200.degree. F. and a wet bulb temperature from about 120.degree. F. to about 180.degree. F. for a time period of less than 12 minutes, and are then further dehydrated with hot air to a moisture content of less than about 8%. The dehydrated food obtained by the above hot air drying technique has the defects as mentioned above.
U.S. Pat. No. 3,984,577 discloses a method of producing a compacted, freeze-vacuum-dehydrated food, e.g., vegetables and meats. This method comprises compacting said food while frozen in particulate form and simultaneously freeze-vacuum-dehydrating said food to a moisture content below about 4.0% by weight, said compacting being carried out by applying mechanical pressures of at least about 20 psi and sufficient to provide a freeze-vacuum-dehydrated food having a bulk density of from about 0.5 to about 1.2 g/cc. In the dehydrating method of the U.S. patent, however, the dehydrating time is long and the dehydrating process is very expensive to carry out.
Japanese Pat. No. 1,087,793 relates to a method of seasoning and dehydrating "aburage". This patent claims a method of seasoning and dehydrating "aburage" which comprises pouring a seasoning liquor at 80.degree. C. or more into the "aburage" under a hot-air atmosphere at about 100.degree. C. or more and simultaneously applying a pulsating microwave radiation. The microwave drying in this patent has defects as mentioned above.
Moreover, in the case where the food to be treated contains a large quantity of a seasoning liquor, (e.g., the seasoned "aburage") and is to be dehydrated by the above drying methods, there are defects that the seasoned food is dehydrated in such state that the seasoning materials are condensed locally in the food to be treated and that the seasoning liquor flows out as a drip from the seasoned food during the drying operation.